Mostly used for Plumbing inside a house. -Personally, I prefer PEX, easier to install and lasts longer.
Water
If the copper is big enough, you can treat it as if it were cast and use packing and sealant. If the diameters are too different, you can use a rubber coupling to connect the two.
Never
You need to give more info than simply "rigid pipe". What kind of "rigid pipe", what pressure will it have and where you are burying it.
You can use copper pipe.
Flare copper pipe and use a flared shutoff valve. If valve isn't flared, use an adapter fitting between copper flared pipe and valve.
No, it is not.
It is not a good idea to use copper pipe with natural gas as any sulphur compounds in the gas can cause an oxide to form in the copper pipe. Often copper is used with propane. In these instances, soft copper tubing is usually used with compression fittings. The tools required are, pipe cutter and 2 wrenches.
It is probably referring to the type of fitting. A fitting that is soldered to a copper pipe. It is called sweating when you use solder and a torch to connect copper pipe together or attach fittings to copper pipe.
Galvanized pipe will be silver/gray color. Copper will be copper. Use a magnet, it'll stick to galvanized pipe but not to copper. Drinking water lines should not be black steel pipe.
Use a lead-free silver solder. It is the only thing that can handle the pressure and bond with the copper pipe.
Yes, if it is rigid PVC conduit approved for electrical installations. Don't use PVC water pipe as it will be rejected if inspected.